A. Field of Invention
The present invention relates to auditory prostheses, including both cochlear implants and hearing aids More specifically, the present invention pertains to an auditory prosthesis in which the natural stapedius muscle reflex is used to modulate automatically the amplification of the audio channel in the system just prior to, and during an interval when the patient utters sounds, as well as when the patient subjectively perceives loud sounds.
B. Description of the Prior Art
The present invention relates generally to auditory prostheses, such as multi-channel cochlear implants and hearing aids. A cochlear implant conventionally consists of three componentsxe2x80x94an implanted electrode array, an implanted receiver/stimulator unit (RSU) and an externally worn speech processor (WP). The speech processor receives ambient sound signals, for example, via a microphone, processes them so as to produce a set of signals corresponding to stimuli, and communicates these signals to the RSU. Communication between the speech processor and the RSU may be provided by an inductive link, a direct cable, or any other suitable means. The RSU, in accordance with the received signals, provides electrical stimulation signals to the electrode array.
Since the patient is exposed to various sound levels an AGC feedback circuit is normally used to keep the level of signals within a predetermined relatively constant range. Typically, this circuitry detects an objective or absolute measure of loudness of the ambient sounds, and not the patient""s subjective perception of the intensity of these sounds. Therefore a problem associated with present cochlear implant systems is that they do not have the capability of responding to the patient""s own subjective perception of sound intensities. This problem is particularly pronounced when a patient starts speaking, because these sounds are subjectively intrinsically louder than ambient sounds but are not treated differently by standard AGC circuitry. This latter situation is somewhat ameliorated by the use in existing systems of unidirectional microphones which are less sensitive to sounds spoken by the patient. However future systems will make use of other schemes, including, for instance, fully implanted systems with implanted microphones. In these fully implanted systems the above mentioned problem will be much more pronounced because implanted microphones are inherently much less directional than the external microphones presently in use and because direct bone conduction will conduct sounds produced by the patient very efficiently to the implanted microphones.
The problem of distinguishing between auditory signals produced by a prosthesis patient and signals produced externally is also present in hearing aids, especially implantable hearing aids. An implantable hearing aid is described, for instance, in U.S. Pat. No. 5,814,095.
The present invention takes advantage of naturally occurring stapedius reflex. This reflex is commonly observed by measuring the acoustic impedance in the middle ear. The acoustic impedance is primarily modulated by the action of the tensor timpani muscle acting on the eardrum, but the action of both the tensor timpani and stapedius muscle is commonly referred to as the stapedius reflex. The following discussion deals with monitoring the stapedius reflex by means of the electrical signal emanating from the stapedius muscle or stapedius nerve. It will be readily understood that the invention can equally be implemented by monitoring the electrical signal emanating from the tensor timpani muscle or tensor timpani nerve.
The stapedius muscle, when contracted, acts as a dampening mechanism on the ossicular chain within the ear. In the normally functioning ear, contraction of the stapedius muscle attenuates the vibration transmitted through the malleus, incus and stapes to the oval window, so as to prevent overstimulation of the auditory system. In a paper by Jerger et al, in Ear and Hearing, vol 9, No 1(1988), entitled xe2x80x9cPrediction of dynamic range for the stapedius reflex in cochlear implant patientsxe2x80x9d, amplitude growth functions for an electrically-elicited stapedius reflex were compared with behavioral estimates of dynamic range. This paper concluded that comfort levels are typically greater than or equal to the saturation or plateau level of stapedius response. The stapedius reflex, whilst electrically elicited, was measured using an external acoustic probe arrangement. The onset of the stapedius reflex is conventionally determined in this manner.
It has been found that most people, even candidates for cochlear implants, have a stapedius reflex. As demonstrated above, this reflex is the intrinsic or natural way in which a body protects itself against loud noises. Importantly, it is the same reflex which is also used to suppress a person""s perception of his own voice.
It has been suggested that the stapedius reflex be used to determine the stimulation comfort and threshold levels of a patient. See, for instance, commonly assigned PCT publication WO97/09863, corresponding to U.S. application Ser. No. 09/029,365 filed Mar. 4, 1998, entitled DERIVED THRESHOLD AND COMFORT LEVEL FOR AUDITORY PROSTHESIS, now U.S. Pat. No. 6,205,360; and PCT International Publication No. WO97/48447 published on Dec. 24, 1997 and entitled SELF-ADJUSTING COCHLEAR IMPLANT SYSTEM AND METHOD FOR FITTING THE SAME. However in both of these references the stapedius reflex is used only to calculate the above-mentioned parameters while the cochlear implant system is fitted. The references do not suggest the use of the stapedius reflex xe2x80x98on the flyxe2x80x99, i.e., during the normal use of a cochlear system to determine a physiological control parameter indicative of the patient""s subjective perception of sound levels, or to detect sounds uttered by or about to be uttered by the patient,
The present inventor has discovered that the stapedius reflex is an ideal parameter to measure a person""s subjective perception of the loudness or intensity of ambient sounds and that this parameter can be used as a gain control parameter during the normal operation of an auditory prosthesis such as a cochlear implant system or a hearing aid. The present inventor has also discovered that this reflex is an ideal indication of when a patient is speaking since the reflex sets in just before a speaking episode.
It is an object of the present invention to provide an arrangement in which a dynamic physiological parameter indicative of the subjective perception to a patient of sound levels is derived automatically in a prosthesis.
A further objective is to provide an auditory prosthesis in which the stapedius reflex is detected and used to control the output signals generated by the prosthesis.
Yet another objective is to provide an auditory prosthesis in which means are provided to differentiate between sounds generated by the patient and ambient sounds.
A further objective is to provide an implantable auditory prosthesis system which selectively and automatically alters the gain of the system in anticipation of a patient""s own voice and subsequent to the resultant utterance.
Yet another objective is to provide an auditory prosthesis system in which the stapedius reflex is used to automatically adjust the system for the perceived loudness of sounds.
Other objectives and advantages of the invention shall become apparent from the following description.
Briefly, the present invention provides an auditory prosthesis which may be, for instance, a cochlear implant including processing means for providing electrical stimulus signals to an implanted stimulation device, said prosthesis including a sensor means adapted to sense the stapedius reflex. The prosthesis also includes a microphone, wherein signals from said microphone are processed by said processing means in accordance with a pre-determined algorithm, so as to define a stimulation sequence to be applied to the patient""s auditory nerve. The intensity of the sounds detected by the microphone is monitored and an AGC circuit is used to maintain said intensity in a predetermined range, the output of the AGC circuit being fed to the processing means. A physiological parameter is determined from the sensor means which is indicative of the subjective perception of the patient of said sound intensity. This physiological parameter is used to adjust the operation of the AGC circuit thereby automatically and dynamically compensating for the sound intensity perceived or anticipated by the patient. Preferably this physiological parameter is derived from the stapedius reflex. An advantage of using the stapedius reflex is that physiological parameter derived from it is indicative of loud ambient sounds and it also comprises a precursor for speech uttered by the patient.
In an alternate embodiment, the physiological parameter is fed to the processing means which modifies its operation in the presence of the physiological parameter. For example, signals received by the signal processing means following the stapedius reflex event are processed differently than signals received prior to the stapedius reflex.
The sensor means for the stapedius reflex may include any means which detects either a tightening of the stapedius muscle itself, or a nervous signal associated with the activation of the muscle. A preferred method is to detect electrically the depolarization of the nerve fiber which initiates the contraction leading to the stapedius reflex. Other methods include measuring the electrical signals generated by the stapedius muscle as it contracts, detecting said contraction mechanically, or detecting consequences of the muscular contraction, such as changes in the acoustic impedance of the middle ear. The sensor means may generate a binary signal indicating the activation of the stapedius reflex, or may be multi-valued indicating the intensity of the reflex.
The principles described above are also applicable to a hearing aid as well, including an external hearing aid or fully implanted hearing aid. In an external hearing aid, the amplified signals, attenuated in the present invention in accordance with the signals received from the stapedius reflex monitor, are fed to an output transducer (known as a receiver) inserted into the patient""s ear. In a fully implanted hearing aid, the amplified signals are fed to a transducer which transmits corresponding vibrations mechanically to the ossicular chain or directly to the footplate.